Passenger planes capable of hypersonic speeds, or five times the speed of sound, have been a dream of engineers for decades. Now, there's a real plan in place to build one. It's just going to take a long time.

The era of supersonic flight came to an ignominious end with the decommissioning of the Concorde in 2003, the end result of one tragic disaster and decades of being unprofitable. Since then, no serious attempts have been made to get planes back above Mach 1, let alone to Mach 5 or higher.

Enter the European Space Agency, which has announced plans to build a hypersonic passenger plane, known as A2, that will fly at least six times faster than any passenger airliner currently in service. The team, which has been given the rather wonderful name of Lapcat, is showing the same sort of ultra long-term thinking that NASA is using for its return to the Moon - at best, Lapcat says their plane will be ready by 2040. By comparison, Concorde only took about a decade from development to first flight.

Admittedly, there are some serious engineering hurdles to be cleared with any hypersonic plane, as the physics for subsonic, supersonic, and hypersonic speeds are all different. Gases and metals in particular can behave very strangely at such speeds, and the engines found in standard passenger planes would not work at Mach 5 or 6. Also, the friction created by air resistance at such speeds would roast the exterior of the plane up to 1,000 degrees Celsius. Solving all these problems will likely take about thirty years, hence the 2040 goal.

Even then, it's an open question whether a hypersonic plane can avoid the financial problems that always dogged Concorde. Most statistics indicate that passengers prioritize cost over speed, and it's unlikely enough passengers will be willing to pay several thousands of dollars to fly from London to Sydney, even if the trip only takes four hours.

Fuel is the big economic issue here. The planes will almost certainly need to use liquid hydrogen as their fuel source, and the amount of the stuff needed to chart ten UK to Australia flight a day would chew up 20% of the United Kingdom's entire power grid. Thirty years is plenty of time to find a cheap way to produce plentiful liquid hydrogen, but if that hurdle can't be overcome, it likely won't make sense to run a hypersonic airline.

That isn't the end of the economic questions, either. Assuming it's based out of Europe, the plane won't be able to flight routes to New York, since the distance is actually too short to reach the high altitudes needed for hypersonic flight. That's problematic because London to New York is the world's busiest business class route, and fares from business class are typically used by airlines to subsidize the relatively cheaper fares in economy.

Much like the Concorde, this may be an instance where the available technology outstrips what we're willing to pay for. Either way, we won't know for certain for another twenty-eight years, at least.